Safety prediction is bedevilled by the philosophical problems of making subjective judgements in the face of limited data, diverse views, political considerations, economic necessity, and issues of responsibility. Safety judgement is a subjective process because it entails the prediction of the likelihood and severity of hazards in the absence of complete foreknowledge. Unanticipated and unprecedented events that could contribute to recognised hazards or pose entirely new ones impose a hidden limit on the level of safety that can be achieved in actuality. Quality of expertise is heavily relied upon. The construction of a safety case is partly a process of rhetorical persuasion; but there are risks in this.